Many types of luminescent devices exist, including a number of all solid state devices. Solid state devices are preferable over incandescent or fluorescent bulbs in that they are lighter; more compact, can be made smaller, and can have higher efficiency. Examples of solid state luminescent devices are light emitting diodes (LEDs), such as gallium arsenide or silicon carbide LEDs, organic light emitting diodes (OLEDs), and polymeric devices typically referred to as polymer light emitting diodes (PLEDs).
Of the various luminescent devices and displays the OLED/PLEDs are the newest and least mature technology. These devices typically consist of a thin film structure comprising a transparent electrode, usually indium doped tin oxide (ITO) on a glass or plastic support layer, the ITO optionally coated with polyaniline or poly(ethylenedioxythiophene) (PEDOT), one or more organic containing layers, typically a hole conducting layer, for example, of a triphenylamine derivative, a luminescent layer, for example, a polyphenylenevinylene derivative or a polyfluorene derivative, an electron conducting layer, for example, an oxadiazole derivative, and a second electrode, for example, calcium, magnesium, aluminum, and the like.
The advantages of the OLED and PLED devices are, lightweight, potentially low cost (although this has yet to be demonstrated commercially), the ability to fabricate thin film, flexible structures, wide viewing angle, and high brightness. The disadvantages of these devices are the short device lifetimes, increasing voltages when operated in a constant current mode, and broad spectral widths. In addition, the efficiency of OLEDs and PLEDs is limited by the nature of the excited state of organic molecules. Typically, both the singlet and triplet excited states are populated during the operation of an OLED/PLED. Unfortunately, in electroluminescence only decay from the singlet state produces useful light. Decay from the triplet state to a singlet ground state is spin forbidden and therefore slow, giving non-radiative processes more time to take place. Because the triplet state is three-fold degenerate and the singlet state is not degenerate; three quarters of the excited electrons enter the triplet state and produce little or no light. (See, A. Kohler, J. S. Wilson, and R. H. Friend, Av. Mater., 2002, 14(10), 701-707.) To address this problem, some groups have attempted to use doped phosphorescent compounds that can utilize both singlet and triplet energies for emission. Exemplary disclosures include, Thompson, et al. U.S. Pat. No. 6,303,238; Thompson, et al. J. Amer. Chem. Soc., 2001, 123, 4304; Lamansky, et al., J. Appl. Phys., 2002, 92(3), 1570; Kim, et al., Appl. Phys. Lett., 2000, 77(15), 2280; and Cao, et al., Appl. Phys. Lett., 2002, 80(12), 2045, the disclosures of which are incorporated herein by reference. Hopwever, all these devices suffer from lower efficiencies compared to small molecule phosphorescent devices, also the long-term stability of these polymeric devices was not reported. It appears that potential phase separation and aggregation hamper the long-term operation of such devices.
An additional disadvantage of OLEDs and PLEDs is the relatively short lifetime of the excited state of organic molecules. In a display application each pixel is scanned 10 to 100 times every second, typically 60 times every second. It is desirable for the light from the pixel to decay on about the same time scale. If the pixel decays too slowly each subsequent image will be scanned over the not yet faded previous image, and the image will blur. If the pixel decays too quickly, there will be a noticeable flicker.
Another inherent problem associated with current OLED/PLED displays arises when one attempts to create white light emitters. LED white light emitters are created by mixing red, green, and blue emitting compounds together, however, because the higher energy blue and green emitters bleed energy into the red emitters when in close proximity, the white light emitters created by simple mixing, have a tendency to produce light with a red tinge. To prevent this “energy bleed” manufacturers have had to physically separate the different colored emitters, such as by creating nanostructure channels. Unfortunately, this additional processing dramatic increases the cost of these white light emitters making them less cost-effective.
As should be understood from the above discussion, there are a number of limitations inherent in conventional OLED/PLED displays that prevent widespread adoption of these devices. Accordingly, there is a need for an improved electroluminescent device that is not limited by the short lifetimes; that has stable I-V characteristics making the associated electronics simpler; that has higher efficiency, not limited by decay from non-luminescent triplet states; that has phosphorescent decay times in the appropriate range for scanned displays and passive displays; that have pure color characteristics that are more amenable to color displays; and/or that have stable white light characteristics.